Abstract Overall, this proposal seeks to define the role of the virome (eukaryotic viruses and bacteriophages) and bacterial microbiome in the health and BK disease of kidney transplant recipients. BK polyomavirus-associated nephropathy (BKVAN) is a major cause of kidney transplant failure. While the mechanistic basis for BKVAN progression is poorly understood, BK viremia is the most consistent surrogate marker for BKVAN. Little else is known about how the virome and bacterial microbiome impacts health and disease in kidney transplantation. In particular, the previously uncharacterized human urinary virome is a significant gap in our understanding of the human microbiome. To address this, we will analyze the circulatory (plasma) virome in a unique longitudinal cohort of kidney transplant recipients and assess their correlates with immunosuppression and BK viremia. Additionally, by defining the urinary virome and bacterial microbiome, we will have an unprecedented opportunity to examine microbiome interactions in the urinary tract. This will allow us to define novel interactions between BK disease and the virome, and explore the mechanisms by which microbiome alterations contribute to the health and disease of kidney transplant recipients. These findings could lead to a better understanding of the early events leading up to BK disease and a novel approach to monitor functional immunocompetence levels through the individual?s virome and microbiome. The mentored K99 phase with Dr. David Wang (Mentor), Dr. Herbert ?Skip? Virgin (Co-mentor) and Dr. Daniel Brennan (Advisor) will support critical training and facilitate the transition to address these questions in a new research field of kidney transplantation.